Thanks for the replies.
I've had another look at the rubber bush between the two upper brackets and found that it's actually split underneath, so that could well be part of the problem.
I assume the rubber is there to reduce vibrations, but does it really need to be rubber? I've got a 3D printer, so I'm wondering if I could make a solid plastic replacement instead.
As for the lower mounting, there is a little side to side movement for adjustment, but I'm not sure whether that's enough to allow the alternator to twist out of alignment.
What seems to happen is that after a few drives, the alternator gradually gets pulled to a slight angle slightly causing the belt to rub and squeal.
Peter, you are probably right I should improve the washer & spacer arrangement to give it a larger surface area to clamp down on the adjustment.
Cheers for suggestions!